A technique of coating a powder with another substance in order to use the powder in various applications is known. With progress in various technical fields, there is a growing desire for a powder having unique properties, especially a metal or metal compound powder. Namely, there is a desire for a powder which combines properties inherent only in a powder, especially a metal or metal compound powder, with other properties so as to have a combination of functions.
For example, in the case of a magnetic metal powder for use as a material for a magnetic color toner, the magnetic powder cannot be used if the color thereof remains as it is, although this does not arouse a problem in conventional black magnetic toners. Any conventionally known coated powder obtained by forming a thin metal oxide film on the surface of a powder for the purpose of surface modification so as to, e.g., protect the powder or facilitate mixing of the powder with, e.g., a synthetic resin does not satisfy the new requirements in such a field. From this standpoint, it is necessary to provide a powder having a novel constitution not seen in any conventional powder.
The present inventors previously invented a powder comprising a base powder particle of either a metal or a metal compound and having thereon a metal oxide film having an even thickness of 0.01 to 20 .mu.m and in which the metal contained therein differs in kind from the metal constituting the base powder particle, in order to provide a powder which combines properties inherent only in the metal particle or metal compound particle with other properties so as to have a combination of functions (JP-A-6-228604). The present inventors further improved the above powder and invented also a powder comprising a base particle having not only a metal oxide film alone but also plural layers of a metal oxide film and a metal film alternately (JP-A-7-90310). Furthermore, the present inventors filed a patent application relating to a process which comprises similarly forming a multilayered metal oxide film on the surface of a metal or metal compound powder and subjecting the powder coated with the multilayered metal oxide film to a heat treatment to produce a powder having a denser and stabler, multilayered metal oxide film (Japanese Patent Application No. 7-80832).
For producing these powders, it is necessary to form two or more metal oxide films each having an even thickness on a base powder particle. Since it is difficult to precipitate either a metal oxide or a metal compound serving as a precursor thereof from an aqueous metal salt solution in forming such a multilayered film, the present inventors developed a method for forming a metal oxide film on a base particle of the powder by dispersing the base particles into a metal alkoxide solution and hydrolyzing the metal alkoxide. Due to this method, it has become possible to form a metal oxide film which is thin and even in thickness and, in particular, it has become possible to form a multilayered metal oxide film.
On the other hand, in the case where a coating material is used for forming a coating film to be used in a high-temperature place, the coating material used is a heat-resistant coating material which withstands the temperature. Also, even when colorant compositions (ink/coating compositions) developed for use in applications where heat resistance or weather resistance is required are used, the coating films obtained by applying the colorant compositions generally have a low heat resistance temperature. This is attributable to the fact that the colorant compositions employ pigments which have low thermal stability at a high temperature. For example, organic pigments suffer discoloration or oxidation/combustion at a temperature not lower than 300.degree. C. Even inorganic pigments, which have relatively high thermal stability, are apt to suffer thermal denaturation and frequently react with additives or adhesive substances to undergo discoloration or color fading. Moreover, long-term exposure to sunlight frequently results in color fading.
A colorant composition generally comprises an inorganic or organic pigment dispersed in a solvent containing dissolved therein a medium (dispersion medium) serving as a film-forming agent. However, none of the prior art pigmented coating materials contains a pigment which itself has a stable color tone at a temperature not lower than 350.degree. C.
In heat-resistant colorant compositions, media (dispersion media) for heat-resistant coating materials have conventionally been thought to be classified by use temperature as follows: organic media and silicone-modified organic media are usable at a temperature up to about 165.degree. C.; silicone-modified aluminum media or silicone media are usable with thermally stable coloring pigments at a temperature up to about 316.degree. C.; silicone media and aluminum pigments are usable or silicone media and black/gray pigments are usable at a temperature up to about 424.degree. C.; silicone media and aluminum pigments are usable at a temperature up to about 538.degree. C.; and silicone-modified aluminum media are usable at a temperature up to about 650.degree. C. It is further thought that at a temperature not lower than 650.degree. C., none of such heat-resistant media is usable and ceramic coating is the only usable technique.
In the case where the film-coated powders described above are to be applied to color printing inks or the like, the powders should be colored. However, since the pigment itself suffers pulverization in a mechanochemical method and comes to have a reduced particle diameter and a lighter color, coloring with a dye or the like is necessary. This applies also to magnetic inks; since the magnetic material has a dull color, a coloring pigment or a white vehicle is added in an exceedingly large amount in order to eliminate the color, resulting in reduced magnetic properties.
In the case of color printing or magnetic color printing for producing gift checks, the demand for which is increasing recently, or producing concert tickets and the like, there are desires not only for elegant coloring and suitability for visual or magnetic reading but also for a special function for forgery prevention.
Furthermore, in production facilities used over long in a high-temperature state, such as high-temperature heat treatment apparatuses, high-temperature reactors, high-temperature dissolvers, and the like, and in general facilities and the like used over long in a high-temperature state, such as high-temperature heaters, cooking utensils, and the like, there frequently is the necessity of coating those parts of such apparatuses and tools which are to be exposed to a high temperature for the purpose of protecting the apparatuses and tools against oxidation or enabling these to retain a beautiful appearance. There is hence a desire for a coating material which does not suffer discoloration/color fading even when exposed to a high temperature over long. In addition, a weather-resistant and light-resistant coating material is desired for the coating of facilities and tools to be used outdoors over long.
Accordingly, an object of the present invention is to eliminate these problems and provide a colorant composition which not only can be used as a color ink of a single, clear, stable color tone, such as blue, green, yellow, or the like, even without incorporation of a dye or pigment thereinto, but also functions to enhance a print forgery preventive performance based on a new technique other than visual examination or magnetic reading when used in combination with a reader. Another object of the present invention is to provide a colorant composition which not only has such excellent functions but also can exhibit an excellent magnetic performance also in magnetic color printing.
Still another object of the present invention is to provide a colorant composition capable of giving a coating film excellent in heat resistance and weather resistance which does not suffer color fading even at a high temperature of 350.degree. C. to 600.degree. C.